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FOUR_OUTPUT_FILTER

2017-02-12 07:19  
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FOUR_OUTPUT_FILTER
The classic state-variable (two-integrator) filter (see Fig. A) is famous for its insensitivity to device parameter tolerances, as well as its ability to provide three simultaneous separate outputs: high pass, bandpass, aind low pass. These advantages often offset the fact that a quad operational amplifier is needed to implernent the circuit.A modification of the classic scheme that applies the input voltage via amplifier U,, rather than UD provides a bandpass output with a fixed peak gain that doesn't depend on the Q of the filter. It was found by using that conftguration, a fourth notch-filter output can be obtained if R1=R6 (see Fig. B).If R1=R6=R12 the gains of both the notch and bandpass outputs are unity, regardless of the Q factor, as determined by R3, R1, R2, R4, R5, and R6. The resonant (or cutoff) frequency is given by ω -1/RO×CO Depending on the capacitor values and frequency ω, resistance RO might also share the same monolithic network for maximum space economy. As with the classic configuration, reso-nant frequency ω can be electrically controlled by switching resistors RO, or by using analog multi-pliers in series with the integrators.